1. Field of the Invention
The present invention relates to filtration equipment using hollow fiber modules, which is used to filter fluids, or liquids, such as condensate water, obtained in a nuclear or thermoelectric power plant, and waste water. More particularly, the present invention relates to filtration equipment comprising a filter vessel, a common upper filtrate collection compartment partitioned with a fluid-tight partition in the filter vessel, a lower filtrate collection casing, and a plurality of hollow fiber modules vertically disposed in the filter vessel between the fluid-tight partition and the lower filtrate collection casing and fluid-tightly joined with the lower filtrate collection casing through respective jointing means while enabling the filtrate to be collected from both the top and bottom ends of the hollow fiber modules.
2. Related Art
In conventional filtration equipment using hollow fiber modules, the hollow fiber modules are simply suspended from a watertight partition, or module support, horizontally disposed in a filter vessel, wherein the filtrate is collected on the upper side of the hollow fiber modules and then withdrawn, or discharged, out of the filter vessel.
More specifically, each of the hollow fiber modules used in the conventional filtration equipment comprises a large number of porous hollow fibers having open top and bottom ends and enclosed in a cylindrical housing having the bottom end thereof covered by a lower header, or lower manifold, defining a lower filtrate collection chamber having no outlet port, and the cylindrical housing has a number of perforations formed through the longitudinal wall thereof for passing feed water therethrough. The filter vessel is partitioned with the watertight partition horizontally disposed therein to form a common upper filtrate collection compartment provided for all the hollow fiber modules and disposed on the upper side of the hollow fiber modules. Feed water entering the cylindrical housings of the hollow fiber modules through their perforations is allowed to permeate porous hollow fibers substantially all over the longitudinal length thereof by means of a hydraulic pressure to thereby effect filtration of the feed water through the walls, or membranes, of the porous hollow fibers into the bores thereof, through which the filtrate is then moved simultaneously downward and upward and then collected in the respective lower filtrate collection chambers of the hollow fiber modules and the common upper filtrate collection compartment of the filter vessel. Every hollow fiber module is provided with a conduit, or central tube, extending from the lower filtrate collection chamber thereof to the common upper filtrate collection compartment. The filtrate collected in the lower filtrate collection chambers of the hollow fiber modules is transported, or shunted, therefrom through the conduits of the hollow fiber modules to the common upper filtrate collection compartment.
Accordingly, all the filtrate is withdrawn, or discharged, out of the filter vessel via the above-mentioned common upper filtrate collection compartment located in the upper part of the filter vessel.
Thus, the foregoing conventional filtration equipment using the hollow fiber modules, though it is of a system wherein the filtrate is collected from both ends of every porous hollow fiber, necessitates the comparatively thin conduits, or central tubes, running from the respective lower filtrate collection chambers of the hollow fiber modules to the top ends thereof to transport, or shunt, the filtrate collected in the lower filtrate collection chambers into the common upper filtrate collection compartment.
This entails addition of a fluid pressure loss through the conduits to the resistance of the membranes of the porous hollow fibers to permeation of water. As a result, the outer membrane surfaces of the upper portions of the porous hollow fibers are more liable to fouling than the outer membrane surfaces of the lower portions of the porous hollow fibers. Hence, it is impossible efficiently to use the porous hollow fibers substantially all over the longitudinal length thereof. Further, since the condiuts are provided inside the respective hollow fiber modules, spaces occupied by the conduits cannot be filled with porous hollow fibers, thus lowering the filtration efficiency of the hollow fiber modules.
A filter vessel designed to solve the foregoing problems was proposed, which comprises upper and lower watertight partitions horizontally disposed in the filter vessel to form upper, intermediate and lower compartments, and a plurality of hollow fiber modules installed in the intermediate compartment, and wherein feed water fed into the intermediate compartment is filtered through the walls of porous hollow fibers in the hollow fiber modules, while the filtrate is collected simultaneously into the upper and lower compartments via the open top and bottom ends of the porous hollow fibers and then discharged from the upper and lower compartments out of the filter vessel (Japanese Patent Laid-Open No. 197,106/1987). In this filter vessel, each hollow fiber module is watertightly joined not only with the upper watertight partition but also with one of lower filtrate collection tubes thrusted through the lower watertight partition by means of a connector to communicate with the lower compartment via the lower filtrate collection tube, and the lower watertight partition is usually welded with the shell plate of the filter vessel to secure the watertight interface therebetween. Thus, removal of the lower watertight partition out of the filter vessel is substantially impossible. This entails a very complicated and costly procedure of inspection and repair of filter vessel internals. More specifically, the above-mentioned procedure involves a series of steps: removal of the upper end plate of the filter vessel, removal of all the hollow fiber modules, removal of the upper watertight partition, inspection and repair of the filter vessel internals, restoration of the upper watertight partition to its place, installation of all the hollow fiber modules, inspection of the jointed state of the hollow fiber modules after installation thereof, and restoration of the upper end plate of the filter vessel to its place. Skilled workmen must enter the filter vessel to inspect and repair therein the lower watertight partition and the lower filtrate collection tubes which cannot substantially be removed out of the filter vessel. Furthermore, the jointed state of the lower part of each hollow fiber module cannot be confirmed and inspected outside the filter vessel because the lower watertight partition fitted with the lower filtrate collection tubes thrusted therethrough cannot substantially be removed out of the filter vessel. This lowers the reliability and certainty of the joints between the hollow fiber modules and the lower filtrate collection tubes.
The foregoing problems may be solved by using a plurality of lower manifolds each provided with at least one conduit communicating with a common upper filtrate collection compartment partitioned with a watertight partition in a filter vessel instead of the lower watertight partition for providing the lower compartment of the foregoing prior art filter vessel (Japanese Patent Application No. 227,259/1993 filed by the assignee of the instant application on Sep. 13, 1993). Each of the lower manifolds independent of the filter vessel is constructed of a plurality of lower filtrate collection tubes, a lower header, and said at least one conduit. Thus, the filtrate from the bottom ends of hollow fiber modules is conducted, or guided, into the common upper filtrate collection compartment via the lower filtrate collection tubes, lower headers and conduits of the lower manifolds to be combined with the filtrate from the upper outlet ports of the hollow fiber modules in the common upper filtrate collection compartment, from which all the filtrate is discharged out of the filter vessel. However, the foregoing structure of filtration equipment involves the following problems. In fabrication, or construction, of each of the lower manifolds, cut tubes of a predetermined length for lower filtrate collection tubes are usually welded with a cut tube of a predetermined length for a lower header, which is also usually welded with at least one cut tube of a predetermined length for a conduit. The inner diameters of these three kinds of cut tubes are determined, having regard to the respective flow rates and permissible pressure losses of the filtrate passed therethrough. Accordingly, as the number of hollow fiber modules for each lower manifold is increased, the diameters of the lower header and the conduit(s) must generally be increased because the flow rate of the filtrate passed therethrough is increased. The inner diameter of the lower header must be larger than that of the lower filtrate collection tubes because the filtrate from the lower outlet ports of the hollow fiber modules for each lower manifold is all combined in the lower header via the lower filtrate collection tubes. The inner diameter of the conduit(s) is usually further larger than that of the lower header. Thus, the structure of the Lower manifolds is inevitably complicated to present a difficult problem from the standpoint of fabrication thereof. Since the lower headers, usually in a tubular form, are horizontally disposed at given intervals, the space secured for the lower headers as flow paths of the filtrate cannot be made the most of in the vertical as well as horizontal direction thereof. Further, the distance between the lower headers must be inevitably increased in proportion to an increase in the diameter of the conduits for securing welding space to enlarge the intervals between the hollow fiber modules, with the result that the diameter of the filter vessel must be uneconomically increased for a given throughput, i.e., the utilization of the space inside the filter vessel is decreased. Furthermore, the configurations of the lower manifolds are so uneven and complicated that a difficulty is encountered not only in fabrication of the lower manifolds themselves but also in fabrication of an air distributor, if any, for providing paths of air bubbles for periodical air scrubbing therewith of porous hollow fibers in the hollow fiber modules. Moreover, since the lower manifolds are fabricated independently of formation of module insertion perforations through the watertight partition, the lower filtrate collection tubes of the lower manifolds are liable to positional deviation in the horizontal direction thereof from the module insertion perforations of the watertight partition. Accordingly, the lower manifolds must be fabricated in such a way that the positions of the lower filtrate collection tubes are so very high in dimensional accuracy as to avoid damage to the hollow fiber modules due to flexure of the hollow fiber modules at the time of installation thereof. Thus, fabrication of the lower manifolds is inevitably uneconomical because of many restrictions imposed thereon.
Accordingly, an object of the present invention is to provide filtration equipment using hollow fiber modules wherein porous hollow fibers can be efficiently used substantially all over the longitudinal length thereof and the filtration efficiency of the hollow fiber modules can be enhanced.
Another object of the present invention is to provide filtration equipment wherein filter vessel internals can be simply and inexpensively inspected and repaired with high reliability and certainty.
Still another object of the present invention is to provide filtration equipment wherein filter vessel internals can be simply and economically fabricated with a high utilization of the space inside a filter vessel.